1. Field of the Invention
The present invention generally relates to charged particle beam systems and, more particularly, to resolution measurement and adjustment in charged particle beam lithography and microscopy tools.
2. Description of the Prior Art
Numerous techniques utilizing charged particle beams are known and in widespread use for manufacture of integrated circuit devices, in particular. For example, charged particle beams are used for implantation of impurities, inspection (e.g. with scanning electron microscopes) of structures for process evaluation and development and for lithographic patterning of substrates and layers deposited thereon. Essentially, lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of the surface for further processing by, for example, etching, implantation and/or deposition. For both lithographic patterning and inspection, electron beams (e-beams) are the charged particle beam of choice since the low mass of electrons allows the beam to be manipulated with high positional accuracy and resolution with reduced energy relative to other particles.
There is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced and noise immunity is increased with reduced connection length and capacitance. Further, increased device density on a chip allows greater chip functionality as well as greater numbers of devices to be manufactured on a chip of a given area; providing increased economy of manufacture if manufacturing yields can be maintained.
Whether for lithographic patterning or inspection, the resolution of the charged particle beam is of paramount importance as integration density is increased in order to expose or discriminate the minimum feature size specified by the design rules for the device. So-called probe-forming systems have been developed to provide a resolution of a small fraction of a micron.
Probe forming systems produce a single minute spot for exposure which must then be deflected through each exposure element of a desired pattern. While deflection can be done at high speed, decreasing feature size multiplies the number of spot exposures which must be made for each integrated circuit device of a given overall area and, thus, increasing integration density reduces throughput below generally acceptable levels to support significant production volume of a single design.
Alternatively, e-beam projection systems (as distinct from probe forming systems) project images of reticle sections or sub-fields which may contain many millions of image elements for a single exposure and throughput is relatively high even at very small feature size and high integration density. The image at the target (wafer) can generally be reduced substantially in size relative to the size of the patterned reticle, generally by a factor of four or five. The beams are thus correspondingly large in cross-section relative to the minimum feature size (whereas the minimum feature size is generally comparable in size to the beam size made by a probe-forming system) and characteristically includes patterned features.
Resolution is generally assessed in regard to probe-forming e-beam systems by scanning the beam across a knife-edge structure provided in the tool for that purpose. Typically, the beam is scanned across the knife edge structure and the rise time of the differentiated back-scatter or transmitted current signal is measured. Dynamic correction elements (e.g. focus coil, stigmator coils, and the like) can be adjusted to optimize resolution. Final adjustments are then made by analysis of the effects of the dynamic corrections on images formed in a resist on a test target (e.g. wafer). It should be appreciated that resolution must be assured over a wide scanning field and portions of the entire resist surface of the wafer are generally employed for such a test. By the same token, correct adjustments over the entire field may not be verifiable in a single test with a single target. Therefore, the adjustment procedure may be protracted and expensive.
However, the knife-edge resolution evaluation technique cannot be extended to e-beam projection systems having a large beam. It can be readily appreciated from the fact that in e-beam projection systems the beam cross-sectional size is on the order of 250 .mu.m (as compared with a resolution of less than 0.1 .mu.m) that extension of the knife-edge technique to such beams would require a knife-edge structure which was smooth, straight and aligned to much less than 0.1 .mu.m over a length of more than 250 .mu.m. Such accuracy would be extremely expensive and may not be possible while, even if available, such a structure would be subject to damage or compromise with each use and in the absence of any convenient or even practical technique to detect such damage or compromise.